Composite ceramic board, method of producing the same, optical/electronic-mounted circuit substrate using said board, and mounted board equipped with said circuit substrate

ABSTRACT

A composite ceramic board comprising an insulating board of insulating layers of alumina ceramics and dielectric layers of ceramics having a dielectric constant smaller than at of said insulating layers which are fired as a unitary structure, and metallized wirings of a low-resistance conductor such as of Au, Ag, Cu or Pt formed on the surfaces and inside thereof, and a method of producing the same. The composite ceramic board not only has a large strength and a high thermal conductivity but also exhibits excellent high-frequency chararteristics and is suited for use as a high-frequency wiring board. The invention further provides an optical/electronic-mounted circuit substrate using the above board, and a mounted board having the circuit substrate of the invention connected to an electronic circuit formed on a mother board.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a composite ceramic board whichhas a plurality of ceramic insulating layers formed as a unitarystructure upon firing, features a large strength, a high thermalconductivity, and is particularly suited as a wiring board forhigh-frequency use, relates to a method of producing the same, to anoptical/electronic-mounted circuit substrate using said board, and to amounted board equipped with said circuit substrate.

[0003] 2. Description of the Related Art

[0004] Accompanying the trend toward highly densely integrating thesemiconductor devices and transmitting signals at high frequencies inrecent years, it has been urged to mount the semiconductor devices on awiring board that has further improved thermal properties and electricalproperties.

[0005] Namely, as the semiconductor devices are highly denselyintegrated, an increased amount of heat is generated from thesemiconductor devices. To prevent the semiconductor devices frommalfunctioning, it is necessary to quickly release the heat out or thedevices. Therefore, the wiring board mounting the semiconductor devicesmust have a high thermal conductivity.

[0006] Further, transmitting the signals at a high frequency bringsabout an increase in the speed of operation, while a delay in thesignals hinders the attempt for increasing the speed of operation.

[0007] To prevent the delay in the signals, therefore, wiring layersmust be formed using a low-resistance conductor having a smallconduction loss.

[0008] Accompanying the recent widespread use of multi-media, on theother hand, it is becoming necessary to transmit and receive tremendousamounts of image data, and attention has been given to opticalcommunication capable of transmitting and receiving large amounts ofdata at high speeds.

[0009] The optical communication device has a structure in whichelectronic semiconductor devices and multi-chip modules are connectedtogether through optical waveguides in a complex manner frequentlyintersecting the waveguides. In order to decrease the size of the deviceby processing optical signals and electric signals using the samemounted substrate and to cope with the complex optical inter-connection,therefore, there has been frequently employed anoptical/electronic-mounted circuit substrate of a structure formingoptical waveguides on the ceramic board and mounting opticalsemiconductor devices and electronic semiconductor devices.

[0010] The optical/electronic-mounted circuit substrate, too, uses theabove-mentioned wiring board, and must have a high heat-radiatingproperty to cope with an increase in the amount of heat generated as aresult of a high degree of integration and high frequencies, and musthave a decreased resistance of the conductor to meet the demand forperforming the operation at high speeds.

[0011] As a wiring board for mounting such semiconductor devices, therehas heretofore been much used a ceramic board obtained by depositingconductor layers (wiring layers) of a high-melting point metal such astungsten or molybdenum on the surfaces or inside of the insulating boardmade of alumina ceramics from the standpoint of reliability.

[0012] However, the above-mentioned conventional ceramic board is notcapable of fully meeting the above-mentioned modern thermal requirementsor electrical requirements.

[0013] For example, the conventional alumina ceramics substrate issatisfactory from the standpoint of thermal properties (thermalconductivity). However, since the wiring layers (conductor layers) havebeen formed of a high-melting point metal, the resistance can be lowereddown to only about 8 mΩ/□ or so. Therefore, signal insertion loss isvery large, and favorable high-frequency characteristics are notobtained.

[0014] Besides, since the insulating board having a wiring layer servingas a terminal for receiving signals is formed of alumina ceramics havinga high dielectric constant, high-frequency signals are reflected to alarge degree and transmission characteristics are deteriorated.

[0015] In order to improve high-frequency signal transmissioncharacteristics by suppressing the reflection of signals, there has beenproposed a wiring board obtained by using glass ceramics having a lowdielectric constant as an insulating layer of the signal input portions,which is formed integrally with a reinforced glass (Japanese UnexaminedPatent Publication (Kokai) No. 239394/1991).

[0016] This wiring substrate is satisfactory from the standpoint ofhigh-frequency characteristics (electric characteristics) since theinsulating layer made of glass ceramic has a low dielectric constant anda conductor such as of copper.

[0017] However, the thermal conductivity of the glass ceramics isseveral watts/m·K at the greatest. Therefore, though high-frequencycharacteristics can be accomplished, heat is not smoothly radiated fromthe semiconductor device, and thermal properties (thermal conductivity)are not satisfactory causing the device itself to malfunction.

[0018] Besides, the insulating board has a small resistance and iscracked at the time of mounting various devices.

[0019] A variety of proposals have heretofore been made to improveproblems related to the above-mentioned thermal properties and electricproperties (high-frequency characteristics) or the conventionalsubstrate.

[0020] For example, Japanese Unexamined Patent Publications (Kokai) Nos.15101/1995 and 151045/2000 disclose wiring boards formed by co-firing aninsulating board made of aluminum oxide and a conductor layer of copperor of a combination of copper and tungsten or molybdenum.

[0021] Further, Japanese Unexamined Patent Publications (Kokai) Nos.106880/1999, 214745/1998 and Japanese Patent No. 3061282 disclose wiringboards equipped with an insulating board having a plurality ofinsulating layers of dissimilar dielectric constants that are formedintegrally together.

[0022] According to Japanese Unexamined Patent Publication (Kokai) No.15101/1995, however, all of the wiring layers (conductor layers) arearranged in the insulating board and are co-fired simultaneously, theinsulating layer on the surface of the insulating board is removed bypolishing so that the inner wiring layer is exposed on the surface ofthe insulating board, or a thick-film method or a thin-film method isapplied onto the surface of the wiring board after firing thereby toform a surface wiring layer (surface conductor layer).

[0023] Therefore, a polishing step, a thick film-forming step and a thinfilm-forming step are indispensable for forming the surface wiring layeraccompanied by such problems as an increased number of the productionsteps, a decreased yield and an increased cost.

[0024] According to Japanese Unexamined Patent Publication (Kokai) No.151045/2000, the firing is conducted at a temperature of not higher than1500° C. and, hence, low-melting point metals such as copper and thelike are separated little, and a conductor layer having a smallresistance is formed.

[0025] Besides, the surface wiring layer (conductor layer) of theinsulating board, too, is formed by co-firing making it possible toavoid an increase in the cost of production.

[0026] However, the insulating layer which is formed of alumina has adielectric constant of as high as about 9.

[0027] In this case, the loss due to the reflection of input signalsincreases in a region where the signals have a frequency of about 40GHz, resulting in a decrease in the characteristics.

[0028] This holds true for the above-mentioned Japanese UnexaminedPatent Publication (Kokai) No. 15101/1995.

[0029] According to Japanese Unexamined Patent Publications (Kokai) Nos.106880/1998, 214745/1998 and Japanese Patent No. 3061282, further, thelayers of low dielectric constants are formed as a unitary structure andthe insulating board is formed of glass ceramics of a composition thatcan be fired at a low temperature. Therefore, signals of highfrequencies can be processed by using a wiring layer (conductor layer)formed of Cu, Au, Ag or Pt having a low resistance as a chief component.

[0030] The boards, however, are not satisfactory in regard to thestrength since the insulating board is made of glass ceramics.

[0031] Even when, for example, a reinforced glass is used, the bendingstrength is about 200 MPa at the greatest.

[0032] Problem further arouses concerning the heat-radiating property(thermal conductivity).

SUMMARY OF THE INVENTION

[0033] It is therefore a first object of the present invention toprovide a composite ceramic board which has a large strength, a highthermal conductivity, exhibits excellent high-frequency characteristics,and is particularly useful as a wiring board for high-frequency use.

[0034] A second object of the present invention is to provide a methodof producing the above composite ceramic board.

[0035] A third object of the present invention is to provide anoptical/electronic-mounted circuit substrate equipped with the abovecomposite ceramic board.

[0036] A fourth object or the present invention is to provide a mountedboard mounting the above-mentioned optical/electronic-mounted circuitsubstrate on an electronic circuit formed on the surface of the motherboard through external connection terminals.

[0037] According to the present invention, there is provided a compositeceramic board wherein insulating layers of alumina ceramics anddielectric layers of ceramics having a dielectric constant smaller thanthat of said insulating layers, are laminated as a unitary structure,and conductor layers containing at least one kind of low-resistanceconductor selected from the group consisting of Au, Ag, Cu and Pt areformed on the surfaced and/or in the inside.

[0038] In the composite ceramic board of the present invention, aninsulating board is formed by the insulating layers of alumina ceramicshaving a high dielectric constant and insulating layers of ceramicshaving a low dielectric constant which are integrally formed together byfiring, to exhibit excellent properties inherent in these insulatinglayers.

[0039] That is, the composite ceramic board includes layers of aluminaceramics that exhibit a high thermal conductivity and large strength andfurther includes ceramic layers having a low dielectric constant. Uponproviding the low-dielectric ceramic layers with an electrode forreceiving external signals, therefore, the loss due to the reflection ofinput signals is effectively decreased and good high-frequency signaltransmission characteristics are accomplished.

[0040] Further, the above insulating board is formed as a unitarystructure by firing at 1200 to 1500° C. Therefore, the conducting layerof a low-resistance conductor such as of copper is formed by co-firingoffering a great advantage from the standpoint of production steps.

[0041] Moreover, since the conductor layer is formed of a low-resistanceconductor such as of copper, the resistance can be decreased andconduction loss can be decreased offering a great advantage intransmitting the high-frequency signals.

[0042] In the composite ceramic board of the present invention, when theinsulating layers and/or the dielectric layers comprise laminates,wiring can be arranged inside the laminates, and a plurality of devicessuch as semiconductors can be mounted being highly densely integrated.

[0043] Further, the dielectric layer is formed at a position to beexposed on the surface of the board, and a conductor layer that servesas an electrode for receiving external signals is formed on the exposedportion. Namely, the conductor layer that serves as a terminal forreceiving signals is formed on the low-dielectric insulating layer tolower the reflection of high-frequency signals input from the wiringlayer thereby to avoid a decrease in the transmission characteristics.

[0044] In order to effectively conduct the co-firing with the conductorlayer containing a low-resistance conductor such as of copper in thepresent invention, furthermore, it is desired that the alumina ceramicinsulating layer contains silica (SiO₂) and Mn₂O₃ in addition to alumina(Al₂O₃) which is a chief component and, particularly, contains from 2 to15% by weight of manganese oxide and from 2 to 15% by weight of siliconoxide.

[0045] It is further desired that the low-dielectric ceramic layercontains, as a chief component, at least one of those selected from thegroup consisting of mullite, forsterite, enstatite, silica andcordierite for it makes easy to cope with higher frequencies.

[0046] It is desired that the conductor layer has a sheet resistance ofnot larger than 8 mΩ/□ calculated as having a thickness of 15 μm and,further, contains at least one kind of a high-melting point metalselected from W and Mo in addition to a low-resistance conductor and,particularly, contains the low-resistance conductor in an amount of from10 to 70% by volume and contains the high-melting point metal in anmount of from 30 to 90% by volume.

[0047] This makes it easy to decrease the resistance of the conductorlayer formed on the surface or in the via-holes of the aluminainsulating layer.

[0048] As a particularly preferred embodiment of the invention, further,there is provided a composite ceramic board wherein the dielectric layercontains forsterite and cordierite as chief crystal phases and, further,contains, as sub-components, at least on of SiO₃, Zn, Mn and alkalineearth metals and/or non-lead·non-alkaline borosilicate glass in an mountof from 0.1 to 20% by weight per the whole amount.

[0049] The composite ceramic substrate of the above embodiment featuresa particularly highly intimate adhesion between the alumina insulatinglayer and the dielectric layer, without warping or cracks of the board,and exhibits very large strength and thermal conductivity as compared tothose of the glass ceramics.

[0050] In particular, upon containing SiO₃ as a sub-component, thefiring can be conducted at a lower temperature, which is desirable.

[0051] Upon setting the composition of the dielectric layer having a lowdielectric constant to be as described above, it is allowed to decreasethe loss due to the reflection of input signals in a high-frequencyregion and, particularly, to decrease the loss in a region of about 60GHz.

[0052] It is particularly desired that the dielectric layer containscordierite in an amount of from 20 to 40% by weight per the wholeamount.

[0053] This decreases the difference in the coefficient of thermalexpansion from the alumina insulating layer, effectively suppresses theoccurrence of peeling from the alumina insulating layer and of cracks,suppresses the dielectric constant of the low-dielectric ceramicinsulating layer to be not larger than 6 to thereby effectively decreasethe signal loss.

[0054] It is further desired that the alumina insulating layer has abending strength of not smaller than 350 MPa.

[0055] This prevents the board from being cracked when the devices arebeing automatically mounted and presents a drop in the yield.

[0056] It is further desired that the alumina insulating layer containsMn in an amount of from 2 to 15% by weight calculated as an oxidethereof, contains Si in an amount of from 2 to 15% by weight calculatedas an oxide thereof, contains at lease one of Mg, Ca, B, Nb, Cr and Coin an amount of from 0.1 to 4% by weight calculated as an oxide thereof,and has a relative density of not smaller than 95%.

[0057] This makes it easy to maintain the strength and the thermalconductivity of the alumina insulating layer.

[0058] According to the present invention, further, there is provided amethod of producing a composite ceramic board of the above-mentionedpreferred embodiment by applying an electrically conducting paste ontolow-dielectric green sheets and onto alumina green sheets containing anoxide powder that contains at least one of Zn, Mn and alkaline earthmetals and/or non-lead·non-alkali borosilicate glass powder for theforsterite powder and the cordierite powder in an amount of 0.1 to 10%by weight per the whole amount, laminating the low-dielectric greensheets and the alumina green sheets, and firing the obtained laminate at1200 to 1500° C.

[0059] According to this method, the alumina ceramics having excellentstrength and thermal conductivity and the dielectric layer having a lowdielectric constant are fired simultaneously, and conductor layers areeasily formed inside the substrate and on the surfaces of the substrate.

[0060] Further, prior to laminating the low-dielectric green sheets andthe alumina green sheets, via-holes are formed in the low-dielectricgreen sheets and/or in the alumina green sheets, and the via-holes arefilled with an electrically conducting paste.

[0061] Thus, a three-dimensional wiring is formed in the ceramics, themulti-layer ceramic substrate is easily realized in a small sizeincorporating such functions as capacitors and inductors.

[0062] It is further desired to prepare the low-dielectric green sheetsby adding the cordierite powder in an amount of from 20 to 40% by weightper the whole amount.

[0063] This makes it possible to match the coefficient of thermalexpansion with that of the alumina green sheets, and to reduce thewarping and cracks during the firing.

[0064] It is further desired that 2 to 15% by weight of Mn₂O₃, 2 to 15%by weight of SiO₂, 0.1 to 4% by weight of at least one of MgO, CaO,B₂O₅, Nb₂O₅, Cr₂O₃ and CoO₃ and the remainder of alumina power are mixedtogether, and are molded to prepare an alumina green sheet.

[0065] This makes it easy to lower the firing temperature while nearlymaintaining the strength and thermal conductivity of the aluminainsulating layer, and to improve the yield of products.

[0066] It is further desired to prepare the electrically conductingpaste by mixing a copper powder in an amount of from 10 to 70% byvolume, and a tungsten powder and/or a molybdenum powder in an amount offrom 30 to 90% by volume. This makes it easy to form a conductor layerof a low resistance even at a firing temperature of from 1200 to 1500°C., which is high than the melting point point of copper.

[0067] According to the present invention, further, there is provided anoptical/electronic-mounted circuit substrate comprising a compositeceramic board of the invention, an optical waveguide and an opticalsemiconductor device mounted on one surface side of the compositeceramic board, an electronic semiconductor device mounted on one surfaceor on the other surface of the composite ceramic board, and an externalconnection terminal provided on the dielectric layer of the compositeceramic board.

[0068] The optical/electronic-mounted circuit substrate of the inventionrealized the optical/electronic-mounted circuit substrate of a smallloss in a portion of receiving high-frequency signals.

[0069] It is particularly desired that the insulating layers and/or thedielectric layers of the composite ceramic board of the invention usedin the above-mentioned optical/electronic-mounted circuit substrate,comprise laminates.

[0070] Therefore, even when a plurality of semiconductors are mounted,the wiring can be arranged even inside the laminate making it easy toaccomplish a highly dense mounting.

[0071] It is further desired that the electronic semiconductor deviceand the optical semiconductor device are mounted on the opposing surfaceof the insulating board.

[0072] This makes it possible to decrease the size and to improve thereliability.

[0073] It is further desired that the electronic semiconductor device iscontained in a cavity formed in the surface of the insulating substrate,and the cavity is air-tightly sealed with a cap.

[0074] This stabilizes the characteristics of the electronicsemiconductor device and improves the reliability thereof.

[0075] It is further desired that the dielectric layer of the insulatingboard is formed on a portion of the surface of the insulating layer.

[0076] This enables a part generating large amounts of heat, such as anelectronic semiconductor device to be mounted on an insulating layerhaving a high thermal conductivity provided with the dielectric layer.

[0077] It is further desired that the optical semiconductor device isprovided inside the optical waveguide.

[0078] This stabilized the characteristics of the optical semiconductordevice and improves the reliability thereof.

[0079] The invention further provides a mounted board wherein anelectronic circuit including capacitors, resistors, and wiringconductors is formed on the surface of a mother board, anoptical/electronic-mounted circuit substrate of the invention is mountedon the electronic circuit via external connection terminals, and areflection loss of when high-frequency signals of 40 GHz are input tothe optical/electronic-mounted circuit substrate is not larger than−10.0 dB.

[0080] This mounted board makes it possible to realize an opticalcommunication which is capable of transmitting and receiving largeamounts of data at high speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0081]FIG. 1 is a sectional view schematically illustrating a compositeceramic board according to an embodiment of the present invention;

[0082]FIG. 2 is a sectional view schematically illustrating a compositeceramic board according to another embodiment of the present invention;

[0083]FIG. 3 is a sectional view schematically illustrating a laminatestructure of a sample board used for the measurement of reflectionlosses in Experiments 1 and 2;

[0084]FIG. 4 is a sectional view schematically illustrating anoptical/electronic-mounted circuit substrate of the present invention;and

[0085]FIG. 5 is a sectional view schematically illustrating a portion ofthe mounted board according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0086] The present invention will now be described with reference to thedrawings.

[0087]FIG. 1 is a diagram schematically illustrating in cross sectionthe structure of a composite ceramic board of the present invention.

[0088] In FIG. 1, the composite ceramic board has an insulating boardgenerally designated at 10. The insulating board 10 comprises aluminaceramic insulating layers (hereinafter simply referred to as aluminalayers) 1 which are thin layers of a sintered body formed chiefly ofalumina, and a ceramic dielectric layer (hereinafter simply referred toas low-dielectric layer) 2 having a dielectric constant smaller thanthat of the alumina layers 1, which are laminated as a unitarystructure.

[0089] Though the composite ceramic board of FIG. 1 has a laminatestructure in which four alumina layers 1 and one low-dielectric layer 2are laminated as a unitary structure, the present invention is in no waylimited to this laminate structure only but may have, as shown in FIG.2, a structure in which the low-dielectric layers 2 are arranged in aplural number (two layers in FIG. 2) (this will be described later).

[0090] On the surfaces and inside of the insulating board 10, further,there are formed metallized wiring layers (conductor layers) 3 a, 3 b, 3c and via-hole conductors 4 as shown in FIG. 1.

[0091] Among these metallized wiring layers, 3 a is a surface wiringlayer, 3 b is a inner wiring layer, and a metallized wiring layer 3 cformed on the surface of the lowermost low-dielectric layer 2 is the oneforming an electrode layer for receiving external signals.

[0092] The composite ceramic board of this structure is easily obtainedby co-firing the alumina layers 1 and the low-dielectric layer 2constituting the insulating board, metallized wiring layers 3 a to 3 c,and electrically conducting paste forming the via-hole conductors 4 at atemperature of from 1200 to 1500° C.

[0093] (Alumina layers)

[0094] The alumina layers 1 are formed chiefly of aluminum oxide andhave a dielectric constant which is not usually smaller than 9.

[0095] It is desired that the alumina layers 1 contain not smaller than84% by weight and, particularly, not smaller than 86% by weight ofaluminum oxide from the standpoint of obtaining a large strength and ahigh thermal conductivity.

[0096] That is, when the amount of aluminum oxide is smaller than theabove range, the strength and thermal conductivity may decrease.

[0097] The aluminum oxide exists as a chief crystal phase of a granularor cylindrical shape. Here, it is desired that the chief crystal phasehas an average crystalline particle diameter of from 1.5 to 5.0 μm.

[0098] When the average crystalline particle diameter of the maincrystal phase is not larger than 1.5 μm, the thermal conductivity may belost to some extent. When the average particle diameter exceeds 5.0 μm,on the other hand, it becomes difficult to obtain a sufficient degree ofstrength.

[0099] When the main crystal phase comprises cylindrical crystals, theabove-mentioned average crystalline particle diameter is based on theshort-axis diameter.

[0100] The alumina layers 1 are formed by the co-firing with themetallized wiring layers 3 a to 3 c that contain the low-resistanceconductor and, hence, must be fired at a temperature of as low as 1200to 1500° C. so as to possess a relative density of, for example, notsmaller than 95%.

[0101] Due to the sintering assistants blended for increasing thedensity, it is desired that the alumina layers 1 contain manganese andsilicon, e.g., contain manganese in an amount of from 2.0 to 15% byweight and, particularly, from 3 to 10% by weight calculated asmanganese oxide (Mn₂O₃) and contain silicon in an amount of from 2.0 to15% by weight and, particularly, from 3 to 10% by weight calculated assilicon oxide (SiO₂).

[0102] That is, when the amount of manganese is smaller than theabove-mentioned range, a high density is not accomplished at 1200 to1500° C. and when the amount of manganese is larger than theabove-mentioned range, then, the insulating property may decrease.

[0103] When the amount of silicon is smaller than the above-mentionedrange, on the other hand, a high density is not accomplished, either,and when the amount of silicon is larger than the above-mentioned range,the thermal conductivity tends to decrease and the dielectric propertytends to be deteriorated.

[0104] In addition to the above-mentioned components, the alumina layers1 may further contain an oxide of at least one kind of element selectedfrom Mg, Ca, Sr, B, Nb, Cr and Co, such as an oxide of an alkaline earthelement like MgO, CaO or SrO in a total amount of from 0.1 to 15% byweight and, preferably, from 0.1 to 4% by weight.

[0105] Namely, use of such compounds as sintering assistants helpsfurther increase the simultaneous sintering with the low-resistanceconductor such as of copper contained in the metallized wiring layers.

[0106] As a coloring component, further, there may be contained a metalsuch as tungsten or molybdenum or an oxide thereof in an amount of notlarger than 2% by weight calculated as a metal.

[0107] The assistants other than aluminum oxide contained in the aluminalayers 1 exist as an amorphous phase or as a crystal phase in the grainboundaries of the chief crystal phase of aluminum oxide. From thestandpoint of increasing the thermal conductivity, however, it isdesired that there has been formed a crystal phase containingassistants.

[0108] From the standpoint of obtaining a high thermal conductivity anda large strength, it is desired that the alumina layers 1 comprising thealuminum oxide and other components have a relative density of notsmaller than 95%, particularly, not smaller than 97% and, moreparticularly, not smaller than 98%, and have a thermal conductivity ofnot smaller than 10 W/m·K, particularly, not smaller than 15 W/m·K and,more particularly, not smaller than 17 W/m·K.

[0109] (Low-dielectric layer)

[0110] The low-dielectric layer 2 has a dielectric constant lower thanthat of the alumina layers 1 and is usually not larger than 8.

[0111] From the standpoint of co-firing with alumina, it is desired thatthe low-dielectric layer 2 contains, as a chief crystal phase, at leastone kind of a low-dielectric oxide selected from mullite, forsterite,enstatite, silica and cordierite in an amount of, for example, notsmaller than 50% by weight and, particularly, not smaller than 70% beweight.

[0112] It is desired that the main crystal phase thereof has an averageparticle diameter which is not larger than 5 μm.

[0113] When the average particle diameter is not smaller than 5 μm,cracks occur in the layer due to sintering and the strength of the boardmay decrease.

[0114] The sintering assistants, coloring components or glass componentsmay be contained in the grain boundaries of the main crystal phase inorder to increase the density under the co-firing conditions.

[0115] In this case, it is desired that the content of the glasscomponents is not larger than 30% by weight, preferably, not larger than25% by weight, more preferably, not larger than 10% by weight and,particularly preferably, not larger than 5% by weight from thestandpoint of maintaining the strength of the board.

[0116] When the amount of the glass components is too great, thelow-dielectric layer 2 that is formed is deformed during the co-firingat the above-mentioned temperature of 1200 to 1500° C. causing thestrength of the board to decrease and causing the glass components todiffuse into the alumina layers 1. As a result, the thermal conductivitydecreases.

[0117] As the sintering assistants and coloring components, there can beexemplified Mn₂O₃, SiO₂, ZnO, CaO, Nb₂O₅, MoO₃ and WO₃, which may becontained in a total amount of from 0.1 to 2% by weight.

[0118] That is, use of these components in combination helps furtherimprove the co-firing with the low-resistance conductor such as ofcopper contained in the metallized wiring layer.

[0119] (Metallized layers (conductor layers))

[0120] In the present invention, the metallized wiring layers 3 a to 3 cand the via-hole conductors 4 contain at least one kind of alow-resistance conductor selected from the group consisting of Au, Ag,Cu and Pt, and, particularly preferably, contains Cu.

[0121] These low-resistance conductors exist as a continuous phase(matrix) to decrease the conductor loss in the metallized wiring layers3 a to 3 c and in the via-hole conductors 4.

[0122] It is further desired that these metallized wiring layers containa high-melting point conductor, such as at least one kind of tungsten(W) or molybdenum (Mo) in order to improve the co-firing property withthe above-mentioned alumina layers and the low-dielectric layer 2 and toenhance the shape-retaining property after the co-firing.

[0123] Concretely speaking, it is desire that the low-resistanceconductor is contained in an amount of from 10 to 70% by volume and,particularly, from 30 to 60% by volume and that the high-melting pointconductor is contained in an amount of from 30 to 90% by volume and,particularly, from 40 to 70% by volume.

[0124] That is, when the amount of the low-resistance conductor such asCu is smaller than the above-mentioned range or when the amount of thehigh melting point metal is larger than the above-mentioned range, themetallized wiring layers exhibit an increased resistance causing theconductor loss to increase.

[0125] Further, when the amount of the low-resistance conductor islarger than the above-mentioned range or when the amount of thehigh-melting point metal is smaller than the above-mentioned range, theshape-retaining property after the co-firing decreases, and the wiringlayers 3 a to 3 c develop oozing, or the wiring layers are aggregatedand broken due to the melting point of the low-resistance conductor suchas of Cu and, besides, the wiring layers may peel off due to adifference in the coefficient of thermal expansion between theinsulating boards 1 and the wiring layers 3 a to 3 c.

[0126] Further, the via-hole conductors 4 become rugged to a largeextent and may escape during the firing.

[0127] It is desired that the above-mentioned high-melting pointconductor such as W or Mo exists in a spherical crystalline state havingan average particle diameter of 1 to 10 μm or in a state in whichseveral particle are bonded together by firing, being dispersed in thematrix of the low-resistance conductor such as of Cu from the standpointof maintaining the resistance low and shape-retaining property. From thestandpoint of resistance of the conductor layers, preventing theseparation and oozing of Cu component, it is particularly desired thatthe high-melting point conductor has an average particle diameter offrom 1.3 to 5 μm and, most preferably, from 1.5 to 3 μm.

[0128] In the present invention, further, the metallized layers maycontain metals such as Ni, Zr, Al, Li, Mg and Zn or oxides thereof,borates thereof, nitrides thereof or carbonates thereof in an amount offrom 0.05 to 3.0% by weight calculated as a metal element.

[0129] The conductor resistance and the co-firing property are adjustedbased upon these metal components.

[0130] It is desired that these metal components have an averageparticle diameter of from 0.06 to 4 μm and, particularly, from 1.5 to3.0 μm.

[0131] In the present invention, further, the low-resistance conductorcomponent in the metallized wiring layers 3 a to 3 c may often diffuseinto the alumina layers 1 and into the low-dielectric layer 2 due to theco-firing at a temperature in excess of the melting point of thelow-resistance conductor such as of Cu. Desirably, here, the diffusiondistance of the low-resistance conductor component and, particularly, ofcopper into the layers is not larger than 20 μm and, particularly, notlarger than 10 μm.

[0132] When the diffusion distance increases, the insulating propertyamong the wiring layers decreases and the reliability of the wiringboard decreases.

[0133] The diffusion distance can be effectively suppressed byconducting the firing in a non-oxidizing atmosphere containing hydrogenand oxygen and having a dew point of not higher than +30° C. and,particularly, from 0 to 25° C.

[0134] The metallized wiring layers 3 a to 3 c and the via-holeconductors 4 having the above-mentioned compositions are so formed as topossess a sheet resistance of not larger than 8 mΩ/□ calculated ashaving a thickness of 15 μm.

[0135] (Layer structure)

[0136] The composite ceramic board of the present invention is equippedwith the insulating board 10 having a layer structure as shown inFIG. 1. Here, however, the layer structure of the insulating board 1 isnot limited to the one shown in FIG. 1 but may be modified in a varietyof ways.

[0137] For example, the insulating board 10 may have a two-layerstructure consisting of the alumina layer 1 and the low-dielectric layer2. Or, as shown in FIG. 2, the insulating board 10 may be of a structurehaving a plurality of low-dielectric layers 2 (two layers in FIG. 2).

[0138] The present invention desirably employs a layer structure inwhich the low-dielectric layer 2 is located at least on the surface (oron the bottom surface) of the insulating board 10, and the wiring layer3 c that serves as a terminal for receiving signals is formed on thelow-dielectric layer 2.

[0139] Namely, upon forming the wiring layer 3 c that serves as aterminal for receiving signals on the low-dielectric layer 2, thereflection is decreased at the time when the high-frequency signals areinput through the wiring layer 3 c to avoid a decrease in thetransmission characteristics.

[0140] It is particularly desired that the ceramic board used for theoptical/electronic-mounted circuit substrate or used for the mountedboard mounting the above circuit substrate on a mother board of thepresent invention that will be described below, employs alumina layers 1and/or low-dielectric layers 2 which are laminates, the dielectriclayers 2 being formed on portions of the surfaces of the alumina layers1.

[0141] Therefore, even when a plurality of semiconductors are mounted,the wirings can be formed even inside the laminates, making it possibleto easily accomplish a high density of mounting. It is further allowedto easily mount a part that generates large amounts of heat such as anelectronic semiconductor device on the alumina layer 1 having a highthermal conductivity which is provided with the low-dielectric layer 2.

[0142] Described below is a preferred embodiment of the compositeceramic board of the present invention that does not permit the board tobe warped or cracked at all during the firing, and features a largestrength, a high thermal conductivity and excellent high-frequencycharacteristics owing to its low-resistance conductor wirings.

[0143] In the composite ceramic board of this embodiment, thelow-dielectric layer 2 contains forsterite and cordierite as chiefcrystal phases.

[0144] Forsterite is necessary for lowering the dielectric constant tobe smaller than that of the alumina layers 1, and cordierite works todecrease the difference in the coefficient of thermal expansion from thealumina layers 1, to reduce the residual stress caused by firing, and tosuppress the warping and cracking of the board.

[0145] The coefficient of thermal expansion of the low-dielectric layer2 is determined by adjusting the contents of forsterite and cordierite.It is desired that the content of cordierite is from 20 to 40% by weightand, particularly, from 25 to 35% by weight.

[0146] It is important that the composite ceramic board of thisembodiment contains, as sub-components, at least one of Zn, Mn andalkaline earth metals and/or a non-lead·non-alkaline borosilicate glass.

[0147] The sub-components promote the reaction with the alumina greensheet during the firing, enhance the adhering strength to the aluminalayers 1, and secures the unitary structure of the alumina layers 1 andthe dielectric layers 2.

[0148] Besides, SiO₂ enables the firing to be conducted at a lowertemperature.

[0149] In the substrate of this embodiment, in particular, it is desiredthat the sub-components are contained in an amount of from 0.1 to 20% byweight, particularly, from 1 to 8% by weight and, more particularly,from 3 to 6% by weight per the whole amount of the low-dielectric layers2.

[0150] This is because, when the amount is smaller than 0.1% by weight,the effect of addition is not obtained to a sufficient degree and whenthe amount exceeds 20% by weight, on the other hand, the liquid phasetends to flow out during the firing.

[0151] It is desired that the alumina layers 1 contain at least one kindof Mg, Ca, B, Nb, Cr and Co in an amount of from 0.1 to 4% by weightcalculated as an oxide thereof in addition to containing alumina,manganese oxide and silicon oxide.

[0152] Use of this composition makes it easy to increase the densityeven at low temperatures and to maintain a large strength and a highthermal conductivity.

[0153] It is desired that the board of this embodiment has a bendingstrength of not smaller than 350 MPa, particularly, not smaller than 400MPa and, more particularly, not smaller than 450 MPa.

[0154] It is desired that conductor layers 3 contain Cu in an amount offrom 10 to 70% by volume and, particularly, from 30 to 60% by volume,and contain W and/or Mo in an amount of from 30 to 90% by volume and,particularly, from 40 to 70% by volume.

[0155] The semiconductor layers 3 having such a composition exhibit asufficiently low electric resistance, maintain intimate adhesion to theconductor layers 3 and/or to the low-dielectric layers 2, and preventsuch inconveniences as peeling of the conductor layers 3, conspicuousruggedness in the surfaces of the via-hole conductors 4, and missing ofvia-hole conductors 4 through the firing.

[0156] It is further desired that the composition further contains atleast one kind of Zr, Al, Li, Mg and Zn in an amount of from 0.05 to3.0% by weight calculated as a metal element in addition to Cu and Wand/or Mo.

[0157] Thus, the conductor layers 3 easily acquire a low resistance, andare intimately adhered to the alumina layers 1 and/or to thelow-dielectric layers 2.

[0158] The composite ceramic board of the present invention constitutedas described above exhibits excellently large strength and thermalconductivity, has little reflection loss of high-frequency signals, andcan be suitably used for the semiconductor packages, electronicparts-mounted circuit substrates and high-frequency wiring boards.

[0159] (Production of ceramic board)

[0160] A method of producing the composite ceramic board of the presentinvention will now be described concerning chiefly the method ofproducing a ceramic board of a preferred embodiment in which thelow-dielectric layer 2 contains forsterite and cordierite as chiefcrystal phases as well as concerning the method of producing ceramicboards of other embodiments.

[0161] First, as the starting material powder for producing the aluminalayers 1, there is used a mixed powder of an aluminum oxide powder,powders of assistant components such as Mn₂O₃, SiO₂ and oxides of Mg,Ca, Sr, B, Nb, Cr and Co, metal powders of transition metals such as W,Mo and Cr and powders of oxides thereof.

[0162] In this case, the assistant components can be used in the form ofcarbonates, nitrates or acetates capable of forming oxides upon firing.

[0163] It is desired that the powder of aluminum oxide has an averageparticle diameter of from 0.5 to 2.0 μm, and particulary, from 0.5 to2.0 μm, so that the chief crystal phase of aluminum oxide that is formedwill assume an average particle diameter that lies within theabove-mentioned range.

[0164] When use is made of a powder having an excessively large averageparticle diameter, it becomes difficult to adjust the average particlediameter of the chief crystal phase to lie within the above-mentionedrange, and, besides, it may become difficult to conduct the firing at atemperature of not higher than 1500° C. as will be described later.

[0165] Even when use is made of a powder having a too small averageparticle diameter, it becomes difficult to adjust the average particlediameter of the chief crystal phase to lie within the above-mentionedrange and, besides, it becomes difficult to handle the powder and thecost of the powder become high.

[0166] The rate of mixing the powder of assistant components is suitablyset to as to satisfy the above-mentioned composition of the aluminalayers 1.

[0167] When, for example, in the case of the board of theabove-mentioned preferred embodiment in which the forsterite andcordierite are forming main crystal phases in the low-dielectric layer,the Mn₂O₃ powder is added in an amount of from 2 to 15% by weight and,particularly, from 3 to 7% by weight, and the SiO₂ powder is added in anamount of from 2 to 15% by weight and, particularly, from 3 to 7% byweight with respect to the aluminum oxide powder.

[0168] It is further desired to add a powder of at least one kind ofMgO, Mg(OH)₂, MgCO₃, CaO, Ca(OH)₂, CaCO₃, B₂O₅, Nb₂O₅, Cr₂O₃ and CoO₃ inan amount of from 0.1 to 4% by weight.

[0169] Since the firing can now be conducted at a low temperature, thereis obtained a highly dense alumina insulating layer, preventing metalsfrom eluting out of the conductor layers 3 and via-hole conductors 4 atthe time of firing yet nearly maintaining the strength and the thermalconductivity, and maintaining a high yield of products.

[0170] The above powdery composition can be further added with a powderof transition metals such as W, Mo and Cr as well as a powder of oxidesas coloring components in an amount of not larger than 2% by weightcalculated as a metal.

[0171] As the starting material powder for producing the low-dielectriclayers 2, there is generally used a mixed powder of a crystalline powderof at least one kind of an oxide having a low dielectric constantselected from mullite, forsterite, enstatite, silica and cordierite, anda powder of assistant components and glass components like thosedescribed above.

[0172] Like the case of the alumina layers 1, the assistant componentscan be used in the form of carbonates, nitrates or acetates capable offorming oxides upon firing.

[0173] The crystalline powder of the oxide having a low dielectricconstant may have an average particle diameter of from 0.5 to 5 μm and,particularly, from 0.5 to 3 μm.

[0174] When the average particle diameter is not smaller than 5 μm, theaverage particle diameter of the chief crystal phase becomes not smallerthan 5 μm, cracks occur in the particles to a conspicuous degree afterthe firing, and the strength may decrease.

[0175] When the average particle diameter is not larger than 0.5 μm, onthe other hand, it may become difficult to handle the powder.

[0176] The powder of assistant components is mixed at such a ratio as tosatisfy the above-mentioned composition of the low-dielectric layer 2.

[0177] In producing the low-dielectric layers 2 in the board of theabove-mentioned preferred embodiment, further, it is important to addthe forsterite powder, cordierite powder and, as sub-components, asoxide powder and/or a non-lead·non-alkaline borosilicate glass powdercontaining at least one of Zn, Mn and alkaline earth metals, in anamount of from 0.1 to 10% by weight, particularly, from 1 to 8% byweight and, more particularly, from 3 to 6% by weight per the wholeamount.

[0178] By adding the sub-components at a ratio of from 0.1 to 10% byweight per the whole amount, the reaction with the alumina green sheetis promoted during the firing, whereby a firm reaction layers is formedand adhesion is accomplished more intimately.

[0179] In order to enhance the sintering property and to decrease theresidual stress during the firing, further, it is desired to add thecordierite powder at a ratio of from 20 to 40% by weight and,particularly, from 25 to 35% by weight.

[0180] Here, the alkaline earth metals which are the sub-components arein the form of powders of oxides of Mg, Ca, Sr and Ba, and the glasspowder is a non-lead·non-alkaline borosilicate glass powder.

[0181] Use is made of the non-lead·non-alkaline borosilicate glasspowder because of the reason that lead causes an extreme burden on theenvironment and the alkali causes a defect in the insulation among thewirings. As such a glass powder, there can be used the one of theSi—Al—B—O type, Si—B—Ca—O type or Si—Al—B—Mg—Zn—O type.

[0182] Here, the forsterite powder and the cordierite powder may be atleast partly substituted by MgO, Al₂O₃, SiO₂ and composite oxide thereofso as to obtain a composition that precipitates forsterite andcordierite.

[0183] In adding the above oxides, further, there may be further addedcarbonates, nitrates or acetates capable of forming oxides upon firingin addition to adding the oxide powders.

[0184] Next, to the above-mentioned starting material powders forproducing the alumina layers 1 and the low-dielectric layers 2, there isadded a suitable amount of an organic solvent such as an organic binderlike polyvinyl alcohol or polyacrylate, or an isopropyl alcohol ortoluene to thereby prepare a slurry for molding. The slurry is thenmolded into a green sheet having a predetermined thickness for formingan alumina layer and into a green sheet for forming a low-dielectriclayer relying upon a known molding method such as doctor blade method,reverse roll coater method, gravure coater method, screen-printingmethod or gravure-printing method.

[0185] In the green sheets are suitably formed through-holes forvia-hole conductors by using a micro-drill or a laser depending upon thelayer structure of the insulating board 10.

[0186] Next, an electrically conducting paste containing a predeterminedlow-resistance conductor and assistant metal components at predeterminedratios, is prepared depending upon the above-mentioned metallizedcomposition.

[0187] In the case of the above-mentioned preferred embodiment, forexample, a copper powder having an average particle diameter of from 1to 10 μm is contained in an amount of from 10 to 70% by weight and,particularly, from 30 to 60% by weight, a tungsten powder and/or amolybdenum powder having an average particle diameter of from 1 to 10 μmis contained in an amount of from 30 to 90% by weight and, particularly,from 40 to 70% by weight and, as desired, at least one kind of Zr, Al,Li, Mg and Zn is contained in an amount of from 0.05 to 3.0% by weightand, particularly, from 0.2 to 2.0% by weight calculated as a metalelement to thereby prepare an electrically conducting paste.

[0188] Here, when the amount of the copper powder is smaller than 10% byvolume, the resistance of the conductor layers 3 becomes high and whenthe amount of the copper powder is not smaller than 70% by volume, itbecomes difficult to maintain the shape during the co-firing of thealumina insulating layers 1, dielectric layers and conductor layers 3,permitting the occurrence of oozing and breakage, or causing theconductor layers 3 to be peeled off the alumina insulating layers 1and/or the dielectric layers 2, and permitting metals to escape from thevia-holes 4.

[0189] In order to enhance intimate adhesion to the alumina layers 1 andto the low-dielectric layers 2, as required, the electrically conductingpaste may be blended with a powder of aluminum oxide, a crystallinepowder of an oxide having a low dielectric constant or a powder of thesame composition as the mixed powder of starting materials used for theformation of alumina layers 1 and the low-dielectric layers 2 in anamount of from 0.05 to 3% by volume.

[0190] The electrically conducting paste prepared as described above isfilled in the through-holes for via-hole conductors formed in the greensheet, and is applied in a pattern of the metallized wiring layers 3 ato 3 c by such a method as screen printing or gravure printing.

[0191] The green sheets onto which the electrically conducting paste isapplied by printing and of which the through-holes are filled with theconducting paste, are positioned depending upon the layer structure ofthe object insulating board 10, laminated, and are press-adhered. Thelaminate is then fired (co-fired) in a non-oxidizing atmosphere at atemperature of from 1200 to 1500° C., preferably, from 1250 to 1450° C.,more preferably, from 1250 to 1400° C. and, most particularly, from 1275to 1350° C. to obtain the composite ceramic board of the presentinvention.

[0192] Here, when the firing temperature is lower than theabove-mentioned range (1200° C.), the alumina layers 1 are not denselyformed to possess a relative density of not smaller than 95% and,besides, the low-dielectric layers 2 are not densely formed, either. Asa result, the obtained board exhibits decreased thermal conductivity andstrength.

[0193] When the firing temperature is not lower than the above-mentionedrange (1500° C.), the high-melting point conductors such as W and Moadded to the conducting paste are sintered, and the homogeneouscomposition of the conductor layers 3 is not longer maintained due tofluidization of low-resistance conductor component such as Cu and,besides, the low resistance is not longer maintained.

[0194] As the non-oxidizing atmosphere at the time of firing, it isdesired to use a nitrogen atmosphere or a mixed atmosphere of nitrogenand hydrogen. In order to suppress the diffusion of low-resistanceconductor such as Cu in the wiring layers, in particular, it is desiredto use a non-oxidizing atmosphere which contains hydrogen and nitrogenand has a dew point of not high than +30° C. and, particularly, from 0to 25° C. as described earlier.

[0195] That is, when the dew point is not lower than −

[0196]30° C. during the firing, the conductor material reacts with thewater in the atmosphere during the firing to form an oxide film, and thealumina layers 1 react with the low-resistance conductor in the wiringlayers interrupting the attempt for lowering the resistance of theconductors and, besides, assisting the diffusion of the low-resistanceconductors.

[0197] As desired, the atmosphere may be mixed with an inert gas such asargon gas.

[0198] The above-mentioned method or producing the composite ceramicboard is capable of simultaneously firing the alumina layers 1,low-dielectric layers 2 and conductor layers 3 having a small electricresistance, and makes it possible to easily realize a composite ceramicpart which has a little loss of signals and is equipped with a highlystrong and highly heat conducting insulating board.

[0199] Next, described below are an optical/electronic-mounted circuitsubstrate of the invention using the above-mentioned composite ceramicboard and a mounted board mounting the above circuit substrate.

[0200]FIG. 4 is a sectional view schematically illustrating theoptical/electronic-mounted circuit substrate of the present invention.

[0201] Namely, there are integrally laminated an alumina layer 1 ofalumina sintered bodies 1 a to 1 d which are thin layers of a sinteredbody formed chiefly of alumina, and a low-dielectric layer 2 having adielectric constant smaller than that of the alumina sintered bodies 1 ato 1 d.

[0202] The alumina layer 1 may be constituted by a single layer ofalumina sintered body. From the standpoint of highly densely mountingthe semiconductors, wirings and decreasing the size, however, it isdesired that the alumina layer 1 is a laminate of a plurality of aluminasintered bodies 1 a to 1 d.

[0203] In FIG. 4, further, though the alumina layer 1 is constituted byfour layers of alumina sintered bodies 1 a to 1 d, there is notparticular limitation on the number of the alumina sintered bodies andtheir number may be suitably determined depending upon the amount ofwiring, positions of the semiconductors and the like.

[0204] Surface conductor layers 3 a and internal conductor layers 3 bare formed on the surfaces or on the interfaces of the alumina sinteredbodies 1 a to 1 d constituting the alumina layer 1, and a groundingconductor layer 3 c is provides between the low-dielectric layers 2 andthe alumina layer 1.

[0205] Via-hole conductors 3 d are formed penetrating through a piece ofor a plurality of pieces of alumina sintered bodies.

[0206] Namely, the conductor layers 3 are formed on the surface and/orinside of the board.

[0207] The constitution described up to this point is the same as theconstitution of the above-mentioned composite ceramic board of thepresent invention.

[0208] In the optical/electronic-mounted circuit substrate of thepresent invention, an optical semiconductor device 5 is mounted and anoptical waveguide 6 is formed on one surface side of the compositeceramic board. The optical waveguide 6 has a structure which isoptically connected to the optical semiconductor element 5, and in whichan optical waveguide clad 6 b is provided surrounding an opticalwaveguide core 6 c.

[0209] The optical waveguide 6 is present in at least a portion on onesurface side of the alumina layer 1. The optical semiconductor device 5may be positioned on the outer side of the optical waveguide 6. In orderto enhance the reliability of the optical semiconductor device 5 and toprevent malfunctioning, however, it is desired that the opticalsemiconductor device 5 is provided on the inside of the opticalwaveguide 6.

[0210] In the optical/electronic-mounted circuit substrate of thepresent invention, further, it is desired that an electronicsemiconductor device 7 is mounted on the other surface of the aluminalayer 1.

[0211] That is, the electronic semiconductor device 7 and the opticalsemiconductor device 5 are mounted on the opposing surfaces of theboard.

[0212] Being constituted as described above, a number of electronicsemiconductor devices 7 and optical semiconductor devices 5 can bemounted on a single board to decrease the size of the product based onthe highly dense mounting and to increase the reliability.

[0213] Here, the low-dielectric layer 2 may be integrally laminated onthe whole surface of the alumina sintered body 1 d constituting theceramic board. As shown in FIG. 4, however, the low-dielectric layer 2may be formed on a portion of the alumina sintered body 1 d.

[0214] Due to this constitution, the surface of the alumina layer 1,too, is exposed on the surface of the ceramic board on the side of thelow-dielectric layer 2 permitting the electronic semiconductor device tobe mounted thereon to enhance the mounting density.

[0215] It is also possible to mount the electronic semiconductor device7 on the surface on one side of the alumina layer 1 on the surface onone side of the alumina layer 1 on where the optical semiconductorelement and optical waveguide are provided.

[0216] Namely, the optical waveguide 6 may be formed on a potion of thesurface, and the electronic semiconductor device 7 may be mounted on thesurface on where the optical waveguide 6 has not been provided.

[0217] The electronic semiconductor device 7 is held in a cavity 8formed in the surface of the board, and is mounted on the surface of thealumina sintered body 1 d by soldering based upon ball mounting and/orbear-chip mounting.

[0218] The cavity 8 is air-tightly sealed with a cap 9, and no externalair enters into the cavity 8. Therefore, the electronic semiconductordevice 7 is prevented from malfunctioning and reliability is improved.

[0219] Moreover, an external connection terminal 101 is provided on thesurface of the low-dielectric layer 2.

[0220] The external connection terminal 101 is for receivinghigh-frequency signals from an external unit, and is electricallyconnected to an external circuit.

[0221] Since the external connection terminal 101 is formed on thelow-dielectric layer 2 having a low dielectric constant, a straycapacity generates little between the external connection terminal 101and the internal conductor layer 3 b, whereby the reflection of inputsignals is suppressed and the signal loss decreases. As a result, thereis realized the optical/electronic-mounted circuit substrate capable ofcoping with high-frequency signals.

[0222] The alumina layer 1 comprises a single or a plurality of aluminasintered bodies.

[0223] In order to increase the mounting density, it is desired to use aplurality of alumina sintered bodies in a laminated form.

[0224] It is further desired that the conductor layers 3 have a sheetresistance of not larger than 8 mΩ/□ calculated as having a thickness of15 μm.

[0225] This low sheet resistance makes it possible to decrease the widthof the wiring and to decrease the size.

[0226] Next, described below is a method of producing theoptical/electronic-mounted circuit substrate of the present invention.

[0227] First, the composite ceramic board of the present invention isprepared by the method described above (production of the ceramicboard), and the optical waveguide is formed on the surface of the board.

[0228] The optical waveguide is formed on the surface of the board asdescribed below. That is, a silica optical waveguide is formed on thealumina insulating layer by a sol-gel method, or an optical waveguide isformed by using an organic material such as polyimide, polymethylmethacrylate or polycarbonate, or an optical waveguide is formed basedon a CVD method.

[0229] Next, the optical semiconductor device is place on the aluminalayer, is electrically connected to a conducting layer of the board andis, at the same time, buried in the optical waveguide clad.

[0230] The optical waveguide is formed on the alumina layer because thelayer forming the optical waveguide is highly strong, the connectionreliability is excellent since the coefficient of thermal expansion ofthe element optically connected to the optical waveguide is close tothat of alumina, and heat generated by the semiconductor device isexcellently conducted.

[0231] Next, the electronic semiconductor device is arranged. Here, itis desired that the electronic semiconductor device is place on thesurface of the insulating layer of alumina. This is because alumina hasa relatively high thermal conductivity unlike glass ceramics and, hence,efficiently and quickly radiates the heat generated by the device.

[0232] Beside, unlike the resin board such as of polyimide, alumina hasa coefficient of thermal expansion close to that of the semiconductordevice and assures a highly reliable primary mounting.

[0233] In this case, the semiconductor device can be mounted on theboard relying upon the ball mounting and/or the bear-chip mounting byusing a solder.

[0234] The mounted board of the present invention has an electroniccircuit including capacitors, resistors and wiring conductors formed onthe surface of the printed substrate such as mother board, and theabove-mentioned optical/electronic-mounted circuit substrate is mountedon the electronic circuit through external connection terminals 101.

[0235] According to the present invention, theoptical/electronic-mounted circuit substrate of the above-mentionedconstitution is used as the mounted board, and the reflection loss ofwhen high-frequency signals of 40 GHz are input to theoptical/electronic-mounted circuit substrate is suppressed to be notlarger than −10.0 dB, particularly, not larger than −7 dB and, moreparticularly, not larger than −5 dB.

[0236] Therefore, use of the optical/electronic-mounted circuitsubstrate makes it possible to receive high-frequency signals of notlower than 40 GHz to cope with the optical communication of a largecapacity.

EMBODIMENTS Experiment 1

[0237] A mixed powder was obtained by adding, to an aluminum oxidepowder (average particle diameter of 1.8 μm), 6% by weight of Mn₂O₃, 6%by weight of SiO₂ and 0.5% by weight of MgO (the amounts of addition areall per the whole amount of the mixed powder).

[0238] To the above mixed powder were further mixed an acrylic binderand toluene to prepare a slurry thereof which was, then, molded into agreen sheet for forming inner insulating layers (correspond to thealumina layers of the invention) having a thickness of 250 μm by thedoctor blade method.

[0239] By using mullite, forsterite, enstatite, silica, cordierite andalumina as chief components, further, the second components wereprepared by mixing the chief components at ratios as shown in Table 1.The mixed powders were then molded into green sheets for forming theouter electrode-forming layer (corresponds to the low-dielectric layerof the present invention) having a thickness of 250 μm in the samemanner as described above.

[0240] As a glass of the second component, there was used a crystallizedglass of borosilicate.

[0241] In Table 1, Mn₂O₃/SiO₂ shown as the second component means thatMn₂O₃ and SiO₂ were used in an equal amount.

[0242] Though-holes for forming via-hole conductors were formed in thethus obtained green sheets at predetermined portions, the through-holeshaving a diameter of 100 to 200 μm after firing. Next, a copper powderhaving an average particle diameter of 5 μm and a tungsten powder havingan average particle diameter of from 0.8 to 12 μm were mixed at a volumeratio of 1:1, and to which were added an acrylic binder and acetone as asolvent to prepare an electrically conducting paste.

[0243] The above electrically conducting paste was applied onto thegreen sheets that have been obtained as described above, and was furtherfilled in the through-holes in the sheets.

[0244] The thus prepared sheets were positioned, laminated andpress-adhered to obtain molded laminate sheets. The laminate sheets weredewaxed in an oxygen-containing atmosphere (H₂+O₂) without substantiallycontaining water, fired in a nitrogen-hydrogen mixed atmosphere having adew point of 20° C. at a temperature shown in Table 1 to obtain wiringboards (samples Nos. 1 to 13).

[0245] The obtained wiring boards were measured concerning the electricresistances of the conductor layers, reflection losses, flexuralstrengths of the boards and thermal conductivities by the methodsdescribed below, and the results were as shown in Table 2.

[0246] The electric resistances of the conductor layers (calculated assheet resistances of a thickness of 15 μm) were measured by afour-terminal method.

[0247] The reflection losses were measured at 40 GHz by using a networkanalyzer and a water probe.

[0248] Concretely speaking, the values were measured between the Teflonsubstrate mounting the sample board and the electrode for measurementprovided on the sample board.

[0249]FIG. 3 is a sectional view illustrating the constitution of thesample being measure (the low-dielectric layer provided with a terminalfor receiving signals possessed a thickness of 0.25 mm, the aluminalayer possessed a thickness of 0.25 mm, the via-hole 4 possessed adiameter of 0.1 mm, the electrode pad 5 (ball pad) possessed a diameterof 0.4 mm, the solder ball 6 possessed a diameter of 0.3 mm, the ballpitch (distance between centers of the neighboring solder balls 6) was0.8 mm, and the Teflon board mounting the sample board possessed athickness of 0.2 mm and a dielectric constant of 3.5).

[0250] The sample board for measuring the flexural strength was adjustedfor its number of the laminated layers so that the low-dielectric layerforming the external electrode possessed a total thickness of 0.5 mm,and alumina layers possessed a total thickness of 2.5 mm, and wasevaluated in terms of a three-point bending strength.

[0251] The thermal conductivity, too, was measured by using the sampleboard having the same laminate structure relying upon the laser flashmethod at room temperature.

Experiment 2

[0252] Wiring boards were produced (samples Nos. 14 to 17 and 20, 21) inthe same manner as in Experiment 1 but changing the electricallyconducting paste (metallizing composition) as shown in Table 1 (when twokinds of conductors were used, the volume ratio was all 1:1), and weremeasured in the same manner as in Experiment 1.

[0253] Further, the wiring boards were produced (samples Nos. 18 and 19)in the same manner as in Experiment 1 but changing the composition ofthe inner insulating layers (correspond to the alumina layers of theinvention) or of the outer electrode-forming layer as shown in Table 1,and were measured in the same manner as in Experiment 1.

[0254] The results were as shown in Table 2. TABLE 1 Innerinsulating-layer Outer electrode-forming layer (Al₂O₃ layer)(low-dielectric layer) Firing Main Amount Main Amount temperatureMetallising No. component (wt %) 2nd component component (wt %) 2ndcomponent (° C.) composition 1 Al₂O₃ 87 Mn₂O₃/SiO₂ Al₂O₃ 87 Mn₂O₃/SiO₂1300 Cu + W 2 Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite 85 Mn₂O₃/SiO₂ 1300 Cu + W 3Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite 80 glass 1300 Cu + W 4 Al₂O₃ 87Mn₂O₃/SiO₂ enstatite 80 glass 1300 Cu + W 5 Al₂O₃ 87 Mn₂O₃/SiO₂enstatite 70 glass 1300 Cu + W 6 Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite 70 SiO₂1300 Cu + W 7 Al₂O₃ 87 Mn₂O₃/SiO₂ enstatite 70 SiO₂ 1300 Cu + W 8 Al₂O₃87 Mn₂O₃/SiO₂ forsterite 70 cordierite 1300 Cu + W 9 Al₂O₃ 87 Mn₂O₃/SiO₂forsterite 60 cordierite 1300 Cu + W 10 Al₂O₃ 87 Mn₂O₃/SiO₂ SiO₂ 60enstatite 1300 Cu + W 11 Al₂O₃ 87 Mn₂O₃/SiO₂ cordierite 70 SiO₂ 1300Cu + W 12 Al₂O₃ 87 Mn₂O₃/SiO₂ SiO₂ 60 cordierite 1300 Cu + W 13 Al₂O₃ 87Mn₂O₃/SiO₂ SiO₂ 85 glass 1300 Cu + W 14 Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite70 cordierite 1300 Cu + Mo 15 Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite 70cordierite 1300 Ag + W 16 Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite 70 cordierite1300 Au + W 17 Al₂O₃ 87 Mn₂O₃/SiO₂ forsterite 70 cordierite 1300 Pt + W*18 forsterite 70 cordierite Al₂O₃ 87 Mn₂O₃/SiO₂ 1300 Cu + W *19forsterite 70 cordierite forsterite 70 cordierite 1300 Cu + W *20 Al₂O₃87 Mn₂O₃/SiO₂ forsterite 70 cordierite 1300 W *21 Al₂O₃ 87 Mn₂O₃/SiO₂forsterite 70 cordierite 1300 Mo

[0255] Dielectric constant Inner Low- Reflection Flexural Thermalinsulating dielectric Sheet loss strength conductivity No. layer Layerresistance (dB) (NPa) (W/m k) Remarks *1 9 9 <8 −6.4 400 16 2 9 8 <8 −10384 15 3 9 7.1 <8 −11.5 380 15 4 9 6.8 <8 −13.6 369 15 5 9 6.6 <8 −14350 15 6 9 6.4 <8 −14.3 376 15 7 9 6.2 <8 −13.5 370 14 8 9 5 <8 −15 38514 9 9 5.8 <8 −15.4 346 13 10 9 5.6 <8 −16 360 14 11 9 5.4 <8 −18 341 1212 9 5.2 <8 −19.7 350 13 13 9 5 <8 −21.9 343 14 14 9 6 <8 −15 385 14 159 6 <8 −15 385 14 16 9 6 <8 −15 385 14 17 9 6 <8 −15 385 14 *18 6 9 <8−6.3 215 3 *19 6 6 <8 −15 197 2 *20 9 6 32 −15 385 14 insertion lossincreased *21 9 6 28 −15 385 14 insertion loss increased

Experiment 3

[0256] As alumina layers, there were mixed an alumina powder (averageparticle diameter of 1.8 μm), and Mn₃O₃ powder, an SiO₂ powder, an MgOpowder, a CaO powder, an SrO powder, a B₂O₅ powder, an Nb₂O₅ powder, aCr₂O₃ powder and a CoO₃ powder in amounts as shown in Table 3, and towhich were mixed, as an organic resin for molding (binder), an acrylicbinder and toluene as a solvent, to prepare a slurry thereof which was,then, molded into a sheet having a thickness of 250 μm by the doctorblade method.

[0257] Via-holes were formed in the sheet at predetermined portions soas to possess a diameter of 100 to 200 μm after firing.

[0258] As low-dielectric layer, further, there were mixed a forsteritepowder, a cordierite powder, a Zn₂SiO₄ powder, an Mn₂O₃ powder, a CaOpowder, an MgO powder, a BaO powder and a non-load·non-alkalineborosilicate glass powder in amounts as shown in Table 3, and to whichwere mixed, as an organic resin for molding (binder, an acrylic binderand toluene as a solvent, to prepare a slurry thereof.

[0259] The slurries were molded into sheets having a thickness of 250 μmby the doctor blade method, and via-holes were formed therein atpredetermined portions so as to possess a diameter of 100 to 200 μmafter firing.

[0260] Next, a copper powder having an average particle diameter of 5 μmand a tungsten powder or a molybdenum powder having an average particlediameter of 5 μm were mixed at ratios shown in Table 3, followed by theaddition of an acrylic binder and acetone as a solution to prepare anelectrically conducting paste.

[0261] Then, the above electrically conducting paste was printed andapplied onto the sheet-like molded articles and was filled in thevia-holes 4 in the sheet-like molded articles.

[0262] The thus produced sheet-like molded articles were positioned,laminated and press-adhered to prepare molded laminates.

[0263] Thereafter, the molded laminates were dewaxed in anoxygen-containing atmosphere without substantially containing water, andwere fired at firing temperatures shown in Table 3 in anitrogen-hydrogen mixed atmosphere having a dew point of 25° C. toproduce composite ceramic boards as shown in FIG. 2.

[0264] The obtained sintered bodies were measured for their specificgravities by Archimedes's method, and relative densities were calculatedfrom the true specific gravities.

[0265] The thus produced composite ceramic boards were observed fortheir warping ad cracks, and were further observed to make sure theappearance of the wiring and via-holes.

[0266] The thermal conductivities were measured based on the laserflashing method in compliance with JIS R 1611.

[0267] The dielectric constants were measured at a frequency of 60 GHzby the cavity resonator method in compliance with JIS R 1627.

[0268] Further, the alumina insulating layers were measured for theirthree-point bending strengths at room temperature in compliance with JISR 1601.

[0269] The contents of forsterite and cordierite were measured based onthe X-ray diffraction and Liedbert's analysis.

[0270] The reflection losses for the signals of 60 GHz were measured byusing a network analyzer and a wafer probe.

[0271] It described in detail, the values were measured between theboard mounting the ceramic board and the electrode for measurementprovided in the ceramic board.

[0272] The results were shown in Tables 3 and 4. TABLE 3 Composition ofComposition of dielectric layer conductor layer Composition ofinsulating layer Alkaline Borosi- High-melting Additive Fors- Cordi-earth metal licate metal Al₂O₃ Mn₂O₃ SiO₂ Content terite erite Mn₂O₃Content Zn₂SiO₄ glass Cu Content No. (wt %) (wt %) (wt %) Kind (wt %)(wt %) (wt %) (wt %) Kind (wt %) (wt %) (wt %) vol % Kind (vol %) 2291.5 5 3 MgO 0.5 75.0 20.0 4.0 CaO 1.0 — — 55 W 45 23 91.5 5 3 MgO 0.562.5 32.5 4.0 CaO 1.0 — — 55 W 45 24 91.5 5 3 MgO 0.5 55.0 40.0 4.0 CaO1.0 — — 55 W 45 25 91.5 5 3 MgO 0.5 64.5 33.5 1.6 CaO 0.4 — — 55 W 45 2691.5 5 3 MgO 0.5 69.2 30.8 8.0 CaO 2.0 — — 55 W 45 27 91.5 5 3 MgO 0.562.5 32.5 8.0 CaO — — — 55 W 45 28 91.5 5 3 MgO 0.5 62.5 32.5 0.0 CaO5.0 — — 55 W 45 29 91.5 5 3 MgO 0.5 62.5 32.5 0.0 CaO — 5.0 — 55 W 45 3091.5 5 3 MgO 0.5 62.5 32.5 0.0 CaO — — 5.0 55 W 45 31 91.5 5 3 MgO 0.564.5 33.5 0.8 CaO 0.2 — 1.0 55 W 45 32 91.5 5 3 MgO 0.5 52.5 32.5 2.4CaO 0.6 — 2.0 55 W 45 33 91.5 5 3 MgO 0.5 59.2 30.8 4.8 CaO 1.2 — 4.0 55W 45 34 91.5 5 3 MgO 0.5 54.5 33.5 0.8 CaO 0.2 1.0 — 55 W 45 35 91.5 5 3MgO 0.5 52.5 32.5 2.4 CaO 0.5 2.0 — 55 W 45 36 91.5 5 3 MgO 0.5 59.230.8 4.8 CaO 1.2 4.0 — 55 W 45 37 91.5 5 3 MgO 0.5 62.5 32.5 4.0 CaO 1.0— — 55 W 45 38 91.5 5 3 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 3991.5 5 3 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 40 91.5 5 3 MgO 0.562.5 32.5 4.0 MgO 1.0 — — 55 W 45 41 91.5 5 3 MgO 0.5 62.5 32.5 4.0 SrO1.0 — — 55 W 45 42 91.5 5 3 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 4397.5 2 0 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 44 89.5 10 0 MgO 0.562.5 32.5 4.0 CaO 1.0 — — 55 W 45 45 84.5 15 0 MgO 0.5 62.5 32.5 4.0 CaO1.0 — — 55 W 45 46 91.5 0 5 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 4789.5 0 10 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 48 84.5 0 15 MgO 0.562.5 32.5 4.0 CaO 1.0 — — 55 W 45 49 91.9 5 3 MgO 0.0 62.5 32.5 4.0 CaO1.0 — — 55 W 45 50 90.0 5 3 MgO 2.0 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 5188.0 5 3 MgO 4.0 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 52 91.5 5 3 CaO 0.562.5 32.5 4.0 CaO 1.0 — — 55 W 45 53 91.5 5 3 B₂O₅ 0.5 62.5 32.5 4.0 CaO1.0 — — 55 W 45 54 91.5 5 3 Nb₂O₅ 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 4555 91.5 5 3 Cr₂O₅ 0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 56 91.5 5 3 CCO₃0.5 62.5 32.5 4.0 CaO 1.0 — — 55 W 45 57 91.5 5 3 MgO 0.5 62.5 32.5 4.0CaO 1.0 — — 10 W 90 58 91.5 5 3 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 40 W60 59 91.5 5 3 MgO 0.5 62.5 32.5 4.0 CaO 1.0 — — 70 W 30 60 91.5 5 3 MgO0.5 62.5 32.5 4.0 CaO 1.0 — — 55 Mo 45  61* 91.5 5 3 MgO 0.5 65.8 34.20.0 — — — — 55 W 45

[0273] TABLE 4 Properties Relative Main crystal Warping Firing densityof phase of Strength of Appearance and Reflection Sample temperatureinsulating dielectric Dielectric insulating of wiring cracks loss No. (°C.) layer (%) layer constant layer (Mpa) via-holes of board (dB) 22 135097 F, C 6.5 472 good no −12.5 23 1350 97 F, C 5.9 472 good no −14.3 241350 97 F, C 5.5 472 good no −15.5 25 1350 97 F, C 6.1 472 good no −14.026 1350 97 F, C 5.8 472 good no −14.8 27 1350 97 F, C 6.2 472 good no−13.5 28 1350 97 F, C 6.0 472 good no −14.0 29 1350 97 F, C, W 6.2 472good no −13.5 30 1350 97 F, C, E 5.8 472 good no −14.8 31 1350 97 F, C,E 6.1 472 good no −13.8 32 1350 97 F, C, E 5.9 472 good no −14.5 33 135097 F, C, E 5.8 472 good no −14.8 34 1350 97 F, C, W 6.1 472 good no−13.8 35 1350 97 F, C, W 5.9 472 good no −14.5 36 1350 97 F, C, W 5.8472 good no −14.8 37 1200 96 F, C 5.8 440 good no −14.8 38 1400 97 F, C6.1 470 good no −13.8 39 1500 97 F, C 6.1 468 good no −13.8 40 1350 97F, C 6.0 472 good no −14.0 41 1350 97 F, C 6.1 472 good no −13.8 42 135097 F, C 6.2 472 good no −13.5 43 1350 93 F, C 6.0 220 good no −14.0 441350 91 F, C 6.0 223 good no −14.1 45 1350 90 F, C 6.0 220 good no −14.146 1350 88 F, C 6.0 170 good no −14.3 47 1350 87 F, C 6.0 150 good no−14.3 48 1350 85 F, C 6.0 133 good no −14.5 49 1350 96 F, C 6.0 489 goodno −14.1 50 1350 97 F, C 6.0 460 good no −13.8 51 1350 97 F, C 6.0 432good no −13.9 52 1350 95 F, C 6.0 456 good no −14.0 53 1350 97 F, C 6.0463 good no −14.3 54 1350 95 F, C 6.0 448 good no −13.0 55 1350 96 F, C6.0 460 good no −13.5 56 1350 96 F, C 6.0 455 good no −13.8 57 1350 97F, C 6.0 472 good no −13.2 58 1350 97 F, C 6.0 472 good no −13.9 59 135097 F, C 6.0 472 good no −14.5 60 1350 97 F, C 6.0 472 good no −14.2 *611350 97 F, C 6.3 472 — yes —

[0274] The boards of samples Nos. 22 to 60 all exhibited good appearanceof wiring and via-holes without warping or cracks, and having reflectionlosses of not larger than −12.5 dB.

[0275] The samples Nos. 61 without sub-components was warped to someextent.

Experiment 4

[0276] In order to form alumina layers, an aluminum oxide powder(average particle diameter of 1.8 μm) was added with 6% by weight ofMn₂O₃, 6% by weight of SiO₂ and 0.5% by weight of MgO, followed by theaddition of an acrylic binder as an organic resin that works as a binderand toluene to prepare a slurry thereof which was, then, molded into agreen sheet having a thickness of 250 μm by the doctor blade method.

[0277] In order to form a low-dielectric layer, further, one or more ofthose selected from mullite, forsterite, enstatite, silica andcordierite were used as chief components and to which were addedadditives shown in FIG. 5 to prepare a sheet-like molded article.

[0278] The dielectric constant of the low-dielectric layer was adjustedby adjusting the amount of silica.

[0279] Via-holes were formed therein at predetermined portions so as topossess a diameter of 100 to 200 μm after firing.

[0280] Next, one of a copper powder having an average particle diameterof 5 μm, and silver powder, a gold powder or a platinum powder, and atungsten powder having an average particle diameter of 0.8 to 12 μm or amolybdenum powder were mixed together at ratios shown in Table 5, and towhich were added an acrylic binder and acetone as a solvent to preparean electrically conducting paste.

[0281] The electrically conducting paste was printed and applied ontothe sheet-like molded articles, and was further filled in the via-holesin the sheet-like molded articles.

[0282] The thus prepared sheet-like molded articles were positioned,laminated and press-adhered to obtain molded laminates.

[0283] The molded laminates were, then, dewaxed in an oxygen-containingatmosphere (H₂+O₂) without substantially containing water, and werefired in a nitrogen-hydrogen mixed atmosphere having a dew point of 20°C. at a temperature 1300° C. to obtain optical/electronic-mountedcircuit substrates shown in FIG. 4.

[0284] The obtained circuit substrates were measured for their densitiesby Archimedes' method.

[0285] The electric resistances of the conductor layers (calculated assheet resistances) in the circuit substrate were measured by afour-terminal method.

[0286] The dielectric constants were measured at a frequency of 40 GHzby the cavity resonator method in compliance with JIS R 1627.

[0287] The reflection losses were measured at 40 GHz by using a networkanalyzer and a wafer probe.

[0288] Concretely speaking, the values were measured between the motherboard 22 mounting the optical/electronic-mounted circuit substrate 21and the electrode for measurement provided in theoptical/electronic-mounted circuit substrate 21.

[0289] The low-dielectric layer having an external connection terminalformed on the surface thereof possessed a thickness of 0.25 mm, thealumina layer possessed a thickness of 0.25 mm, and the via-holepossessed a diameter of 0.1 mm.

[0290] The electrode pad (ball pad) constituting the external connectionterminal possessed a diameter of 0.4 mm, the solder ball possessed adiameter of 0.3 mm, the ball pitch was 0.8 mm, and the Teflon boardhaving a thickness of 0.2 mm and a dielectric constant of 3.5 was usedas the mother board for mounting the optical/electronic-mounted circuitsubstrate.

[0291] The ceramic layer was measured for its flexural strength by usinga co-fired sample having a low-dielectric insulating layer of athickness of 0.5 mm which was for forming an outer electrode and aninsulating layer, other than the above insulating layer, of a thicknessof 2.5 mm.

[0292] The thermal conductivity of the ceramic layer, too, was measuredby using the sample having the laminate structure used for themeasurement of flexural strength relying upon the laser flash method atroom temperature. TABLE 5 Low-dielectric layer Composition of conductorProperties Main layer Thermal Dielectric component Main componentFlexural conduc- constant Sheet Reflection Amount Amount Amount Densitystrength tivity Insulating Dielectric resistance Loss No. Kind (wt %)Additive Kind (wt %) Kind (wt %) (g/cm³) (MPa) (W/mK) layer layer (πΩ/Π)(dB) *62 A 87 Mn₂O₃/SiO₂ Cu 40 W 60 3.7 400 16 9 9 5 −6.4 63 F 85Mn₂O₃/SiO₂ CU 40 W 60 3.5 384 15 9 8 5 10 64 F 80 glass Cu 40 W 60 3.4380 15 9 7.1 5 −11.5 65 E 80 glass Cu 40 W 60 3.4 369 15 9 6.8 5 −13.666 E 70 glass Cu 40 W 60 3.3 350 15 9 6.6 5 −14 67 F 70 SiO₂ Cu 40 W 603.3 376 16 9 6.4 6 −14.3 68 E 70 SiO₂ Cu 40 W 60 3.3 370 14 9 6.2 6−13.5 69 F 70 C Cu 40 W 60 3.3 385 14 9 6 5 −15 70 F 60 C Cu 40. W 603.2 346 12 9 5.8 5 −15.4 71 S 60 E Cu 40 W 60 3.2 360 14 9 5.6 5 −15 72C 70 SiO₂ Cu 40 W 60 3.3 341 12 9 5.4 6 −18 73 S 60 C Cu 40 W 60 3.2 35013 9 5.2 5 −19.7 74 S 85 glass Cu 40 W 60 3.5 348 14 9 5 6 −21.9 75 F 70C Cu 40 Mo 60 3.3 386 14 9 4.8 6 −15 76 F 70 C Ag 40 W 60 3.3 385 14 94.8 5 −15 77 F 70 C Au 40 W 60 3.3 386 14 9 4.6 5 −15 78 F 70 C Pt 40 W60 3.3 385 14 9 4.4 6 −15 79 M 70 C Cu 40 W 60 3.3 370 14 9 4.4 6 −15*80 A 70 Mn₂O₃/SiO₂ W 100 — — 3.6 385 14 9 9 32 −6.5 *81 A 70 Mn₂O₃/SiO₂Mo 100 — — 3.6 385 14 9 9 28 −6.5

[0293] In the samples Nos. 68 and 84 of the present invention, thelow-dielectric layers exhibited dielectric constants of 4.4 to 8, whichwere smaller than the dielectric constant 9 of the alumina layer, andexhibited reflection losses of not larger than −10 dB.

[0294] The sample No. 67 (not included in the scope of the invention) inwhich the alumina layer and the low-dielectric layer were formed of thesame material, exhibited a reflection loss of −6.4 dB.

[0295] The samples Nos. 85 and 86 in which the conductor layers wereformed of W or Mo lying outside the scope of the present invention,exhibited sheet resistances of not smaller than 28 mΩ/□ and reflectionlosses of as large as −6.5 dB.

What we claim is:
 1. A composite ceramic board wherein insulating layersof alumina ceramics and dielectric layers of ceramics having adielectric constant smaller than that of said insulating layers, arelaminated as a unitary structure, and conductor layers containing atleast one kind of low-resistance conductor selected from the groupconsisting of Au, Ag, Cu and Pt are formed on the surfaces and/or in theinside.
 2. A composite ceramic board according to claim 1, wherein saidinsulating layers and/or said dielectric layers comprise laminates.
 3. Acomposite ceramic board according to claim 1, wherein said dielectriclayer is formed at a position to be exposed on the surface of the board,and a conductor layers that serves as an electrode for receivingexternal signals is formed on the exposed portion.
 4. A compositeceramic board according to claim 1, wherein said insulating layersfurther contain manganese and silicon in the form of oxides.
 5. Acomposite ceramic board according to claim 1, wherein said insulatinglayers contain from 2 to 15% by weight of manganese oxide and from 2 to15% by weight of silicon oxide, respectively.
 6. A composite ceramicboard according to claim 1, wherein said dielectric layer contains, as achief component, at least one kind of oxide crystals selected from thegroup consisting of mullite, forsterite, enstatite, silica andcordierite.
 7. A composite ceramic board according to claim 1, whereinsaid conductor layers contain at least one kind of a high-melting pointmetal selected from the group consisting of tungsten and molybdenumtogether with said low-resistance conductor.
 8. A composite ceramicboard according to claim 7, wherein said conductor layers contain saidlow-resistance conductor in an amount of from 10 to 70% by volume andcontains said high-melting point in an amount of from 30 to 90% byvolume.
 9. A composite ceramic board according to claim 1, wherein saidconductor layers have a sheet resistance of not larger than 8 mΩ/□calculated as having a thickness of 15 μm.
 10. A composite ceramic boardaccording to any one of claims 1 to 9, wherein said dielectric layercontains forsterite and cordierite as chief crystal phases and, further,contains, as sub-components, at least one of SiO₂, Zn, Mn and alkalineearth metals and/or non-lead·non-alkaline borosilicate glass in anamount of from 0.1 to 20% by weight per the whole amount.
 11. Acomposite ceramic board according to claim 10, wherein said dielectriclayer contains cordierite in an amount of from 20 to 40% by weight perthe whole amount.
 12. A composite ceramic board according to claim 10,wherein said insulating layer has a bending strength of not smaller than350 MPa.
 13. A composite ceramic board according to claim 10, whereinsaid insulating layer contains manganese in an amount of from 2 to 15%by weight calculated as an oxide thereof, contains Si in an amount offrom 2 to 15% by weight calculated as an oxide thereof, contains atleast one of Mg, Ca, B, Nb, Cr and Co in an amount of from 0.1 to 4% byweight calculated as an oxide thereof, and has a relative density of notsmaller than 95%.
 14. A method of producing a composite ceramic board ofany one of claims 10 to 13, comprising applying an electricallyconducting paste onto low-dielectric green sheets and onto alumina greensheets containing an oxide powder that contains at least one of SiO₂,Zn, Mn and alkaline earth metals and/or non-lead·non-alkali borosilicateglass powder for the forsterite powder and the cordierite powder in anamount of 0.1 to 20% by weight per the whole amount, laminating saidlow-dielectric green sheets and said alumina green sheets, and firingthe obtained aminate at 1200 to 1500° C.
 15. A method of producing acomposite ceramic board according to claim 14, wherein, prior tolaminating said low-dielectric green sheets and said alumina greensheets, via-holes are formed in said low-dielectric green sheets and/orin said alumina green sheets, and said via-holes are filled with anelectrically conducting paste.
 16. A method of producing a compositeceramic board according to claim 14, wherein said low-dielectric greensheet is prepare by adding said cordierite powder in an amount of from20 to 40% by weight per the whole amount.
 17. A method of producing acomposite ceramic board according to claim 14, wherein 2 to 15% byweight of Mn₂O₅, 2 to 15% by weight of SiO₂, 0.1 to 4% by weight of atleast one of MgO, CaO, B₂O₃, Nb₂O₅, Cr₂O₃ and CoO₃ and the remainder ofalumina power, and mixed together, and are molded to prepare an aluminagreen sheet.
 18. A method of producing a composite ceramic boardaccording to claim 14, wherein said electrically conducting paste isprepared by mixing a copper powder in an amount of 10 to 70% by volume,and a tungsten powder and/or a molybdenum powder in an amount of 30 to90% by volume.
 19. An optical/electronic-mounted circuit substratecomprising: a composite ceramic board of any one of claims 1 to 13,wherein insulating layers of alumina ceramics and dielectric layers ofceramics having a dielectric constant smaller than that of saidinsulating layers, are laminated as a unitary structure, and conductorlayers containing at least one kind of low-resistance conductor selectedfrom the group consisting of Au, Ag, Cu and Pt are formed on thesurfaces and/or in the inside; an optical waveguide and an opticalsemiconductor device mounted on one surface side of said compositeceramic board; an electronic semiconductor device mounted on one surfaceor on the other surface of said composite ceramic board; and externalconnection terminals provided on said dielectric layers of saidcomposite ceramic board.
 20. An optical/electronic-mounted circuitsubstrate according to claim 19, wherein said electronic semiconductordevice and said optical semiconductor device are mounted on the opposingsurfaces of said insulating board.
 21. An optical/electronic-mountedcircuit substrate according to claim 19, wherein the electronicsemiconductor device is contained in a cavity formed in the surface ofsaid insulating substrate, and said cavity is air-tightly sealed with acap.
 22. An optical/electronic-mounted circuit substrate according toclaim 19, wherein said dielectric layer of said composite ceramic boardis formed on a portion of the surface of said insulating layer.
 23. Anoptical/electronic-mounted circuit substrate according to claim 19,wherein said optical semiconductor device is provided inside saidoptical waveguide.
 24. A mounted board in which an electronic circuitincluding capacitors, resistors and wiring conductors is formed on thesurface of a mother board, an optical/electronic-mounted circuitsubstrate according to any one of claims 19 to 23 is mounted on saidelectronic circuit via external connection terminals, and a reflectionloss of when high-frequency signals of 40 GHz are input to saidoptical/electronic-mounted circuit substrate is not larger than −10.0dB.